1. Technical Field
The present disclosure relates to a negative electrode active material, an electrode containing the negative electrode active material, and a lithium ion secondary battery having the electrode.
2. Related Art
Lithium ion secondary batteries are lighter in weight and higher in capacity than nickel-cadmium batteries, nickel-metal hydride batteries, and the like. The lithium ion secondary battery has been therefore widely used as a power source for a mobile electronic appliance. Further, as the mobile electronic appliances decrease in size and increase in functionality in recent years, the lithium ion secondary battery has been expected to have further higher capacity. Not just for the mobile electronic appliances, furthermore, the high-capacity lithium ion secondary battery has been a strong candidate as a power source to be mounted on a hybrid vehicle, an electric vehicle, or the like.
The capacity of the lithium ion secondary battery mainly depends on an active material of an electrode. In general, for example, a negative electrode active material contains graphite. However, for meeting the above demand, a negative electrode active material with higher capacity has been desired. Therefore, silicon has attracted attention as a material for the negative electrode active material. Silicon has much higher theoretical capacity (4210 mAh/g) than the theoretical capacity (372 mAh/g) of graphite.
In a negative electrode active material including a mixture of silicon and silicon oxide, the stress caused by expansion and shrinkage of silicon during charging and discharging is relaxed by silicon oxide. The mixture is therefore considered to have superior cycle characteristics to silicon. However, the mixture of silicon and silicon oxide has poor electric conductivity. As a result, when the current density during discharging is high relative to the battery capacity, the use of this mixture as the negative electrode active material causes a significant decrease in discharge capacity. On the other hand, the power source for a hybrid vehicle and an electric vehicle is desired to have high discharge capacity at high rate. Therefore, the use of this mixture as the material of the negative electrode active material of such a power source has a problem.
JP-A-2001-15101 and JP-A-2004-55505 suggest techniques of covering silicon and silicon oxide with carbon in order to increase the discharge capacity at high rate. However, in the techniques disclosed in these documents, a surface of the negative electrode active material is mostly covered with carbon, so that the contact area between the negative electrode active material and an electrolyte solution becomes small. Hence, these techniques fail to increase the discharge capacity at high rate sufficiently.